DESCRIPTION (Adapted from the Investigator's Abstract) The cause of Idiopathic Parkinson's disease (PD), a debilitating disease that afflicts an estimated 1 percent of persons age 60 or older, remains unknown. Research by investigators affiliated with this project and by others have demonstrated a significant familial component in the risk for PD. This proposal is in response to a Program Announcement from the National Institute of Neurological Disorders and Stroke, requesting research into possible genetic factors in the cause of Parkinson's disease. The goal of this project is to begin linkage studies to map genes which predispose to PD risk. To identify regions of the genome containing genes associated with PD risk, the investigators propose a 10 centimorgan density genome scan in 400 affected sibling pairs. Participants will be recruited through a multi-institutional collaboration of ten affiliated clinical sites. The 400 affected sib pairs will consist of two sets of 200 pairs each. The genome scan will be done separately in each set of 200, with the second set providing a mechanism for pursuing promising linkages and replicating significant linkages found in the first set. The investigators will assess possible genetic heterogeneity associated with variation in age at onset, PD family history, and risk factor involvement.